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About the Author History Natural Wetlands Human Use Constructed Wetland Designs Plants and Planting Animals Microbes Biogeochemical Cycling Greenhouse Gases Conclusion References |
There
are three typical basic designs that are applied to the construction
of wetlands. Free surface flow (FSW)  Surface flow systems are modeled most closely on a natural wetland system of the three systems discussed here. Basins or channels of wastewater are open to the atmosphere and soils are constantly submerged to allow for rooting of emergent and submerged macrophites. These systems are predominantly utilized for storm water treatment or tertiary or quaternary treatment of other types of wastewater. Subsurface horizontal flow ( HF)  Subsurface horizontal flow wetlands consist of a water impermeable lined basin containing a gravel or sand fill. The basin has a sand layer and a slope of approximately 1% from inlet to outlet to facilitate gravity feed of the influent to the outlet. The lining for the wetland is most often made of a synthetic material such as HDPE or LDPE with a thickness of 2mm; alternatively a layer of packed clay may be used instead of a synthetic liner. The gravel or sand fill should be adequately coarse to allow the influent to circulate freely while providing a substrate for a substrate for microbial biofilm development and chemical adsobtion of pollutants. The fill also allows for the rooting of hydrophitic vegetation. The wetland can be mulched with a compost layer to aid in the sequestration of noxious gasses, and reduce the unpleasant smells often associated with wastewater treatment. Supplemental aeration lines may be added to the bottom of the wetland to facilitate heterotrophic oxidation of contaminants by microbes.Vertical flow (VF)  Vertical subsurface flow wetlands are constructed in the same manner as the SSHF with the caveat that the fill is layered with sand at the top and becoming progressively larger in diameter with depth ending with a layer of stone at the bottom. The graduation of fill size allows the influent that is pumped or siphoned over the top of the wetland to percolate freely through the system increasing the rate of oxygenation. The influent is pumped or siphoned discontinuously and intermittently over the surface of the wetland to allow times of oxic and anoxic microbial respiration to facilitate remediation of the influent. This type of system maybe planted, but due to the top down flow of the influent the surface of the system should not be mulched. This system is less successful in climates that experience cold temperatures during the winters due to freezing and should be avoided in zones that experience cold winters. The surface of the system may also be prone to clogging with organic matter from the influent, and must be monitored for pooling of the influent. Hybrid systems Each of the three systems may be utilized in conjunction with the others (within appropriate climates). Creating hybrid systems helps to ameliorate the shortcomings of a single unit and enhancing specific microbial populations and redox chemistry to perform remediation tasks in each successive treatment system (more on biogeochemistry and Microbes).  Creating a Hybrid system with a virtical flow component then a subserface flow system alows for airobic nitrification then anairobic denitrification.  Multi stage systems allow for the greatest range of microbial process to perform remediation and gives cleaner effluent. The verity of anoxic subsurface flow, vertical flow components with discontinuous flow providing oxic periods punctuated with brief times of anoxia, and free surface water systems provides the greatest range of microbial habitat and also allows the introduction of many different plant species to provide a more diverse wild life habitat that is inherently more pleasing to view. All images from http://www.iridra.it/index_eng.htm |
| UVM NR260:
Wetlands Ecology and Management Contact: mcunning@uvm.edu, University of Vermont Page created/updated: 02/10/2006 |